The present invention is directed to a fire detection system for use with a simple, two-wire arrangement in which spurious alarm signals are avoided. In addition certain functions, such as an evacuation alarm, signalling the fire department, releasing fire extinguishing material, and so forth, can be sequentially initiated responsive to sucessive indications from different detectors.
One early fire detection system connected a plurality of detectors in parallel across the same conductor pair, normally energized by a 24 volt potential difference. A control unit produced the energizing d-c potential difference from an a-c energizing voltage, and the control unit actuated one or more associated pieces of equipment when an individual detector "alarmed", or produced a fire-indicating signal. The "alarmed detector" placed a virtual short circuit across the 24 volt line at the detector location and therefore prevented receipt of subsequent alarm information from other detectors.
This early system was improved by a system termed "cross zoning". In this arrangement all the detectors were not connected in the same loop, but alternate detectors were connected in one loop, and the remaining detectors then connected in another loop. With this arrangement, a first signal is provided from the first loop when any detector on that loop is actuated. A second signal from the other loop is required before extinguishing material (such as Halon 1301) is released to extinguish the fire. This provides an advantage over the earlier system in that the second loop signal "confirms" the presence of a spreading fire condition.
Another detection arrangement, which may be termed "adjacent zoning", has also been developed. In this system there are no pre-defined zones or loops, but when the first detector is alarmed, that detector is established as the center of a zone including all adjacent detectors. If one of those adjacent detectors is then subsequently alarmed, this confirms the signal from the first detector and is used to initiate some action. However, this system requires a dedicated conductor for each detector, in addition to those across which the detectors are normally connected to transmit signals back and forth for establishing a temporary zone in the area of the first-alarmed detector. The additional conductor for each detector thus adds to the complexity and expense of the system.
It is therefore a primary object of the present invention to provide an improved fire detection system which achieves virtually the same benefits as the various zoning schemes, without the necessity of adding additional wires to the system.
Another important object of the invention is to provide such a system with the capability of effecting protective actions in sequence, as successive detectors are alarmed.